Specific substances present in the blood may be associated with specific diseases. For example, high levels or low levels of the specific substances out of a certain range may indicate development or seriousness of the specific diseases, and therefore such substances have been intensively measured in the field of clinical diagnosis.
There is homocysteine measurement in the blood, for example. Homocysteine is one of intermediate substances produced in the process of methionine metabolism in the body. Produced homocysteine is quickly metabolized through either of the pathways; conversion into methionine, or conversion into cysteine via cystathionine formation. High levels of homocysteine are considered as one of risk factors for cardiovascular disorders and thus the measurement thereof has attracted attention.
Immunological assay method is known as a method for measuring substances such as proteins, steroids, vitamins, and other immunologically antigenic substances (substances capable of artificially producing antibodies against the substances) in the blood. As the immunological assay method for homocysteine (Patent Literature 1), what is called a competitive assay method has been known. In the competitive assay method, the blood is used as a sample and homocysteine bound to the component in the blood by the S—S bond is first released. And next, homocysteine is converted into S-adenosylhomocysteine which is an immunologically measurable derivative, by addition of adenosine with the enzyme. Subsequently, this S-adenosylhomocysteine is competitively bound to a labeled anti-S-adenosylhomocysteine antibody with S-adenosylhomocysteine which was prepared in advance and immobilized on a water-insoluble solid phase. Next, homocysteine was measured by measurement of the amount of the labeled antibody bound to the solid phase after what is called a B/F (bound/free) separation procedure.